When you set as "optional" an area containing subareas it triggers a NullPointerException.

This error is triggered because even if the area is optional, the template script is evaluated.
In the template script you can find call to the subareas [@cms.area name="TheSubArea"/] and as the parent area is optional, no content is created, then this call trigger a nullpointerException.

A solution could be to not evaluate a script of an optional Area, like when you disable an area. But the difference is that the edit bar has to be created.